Fig. 1. Two types of lithium niobate material. (a) Lithium niobate crystal^［32］; (b) single-crystal thin-film lithium niobate wafer^［40］

Fig. 2. Fabrication process of single-crystal thin-film lithium niobate wafer. (a) Ion implantation; (b) substrate preparation; (c) wafer bonding; (d) annealing and splitting; (e) annealing and polishing

Fig. 3. Different types of lithium niobate modulators. (a) Traditional diffused lithium niobate modulator; (b) rib-loaded lithium niobate modulator; (c) heterogenous-bonded lithium niobate modulator; (d) monolithic thin-film lithium niobate modulator

Fig. 4. Overall structure diagram of the development of thin-film lithium niobate modulator

Fig. 5. Non-resonator types of thin-film lithium niobate modulators with different structures. These types of modulators include Mach-Zehnder interferometer based modulators using regular traveling-wave electrode (TWE), Mach-Zehnder interferometer based modulators using capacitively loaded traveling-wave electrode (CLTWE), and single/dual-polarization IQ modulators

Fig. 6. MZI-based modulators using regular traveling-wave electrode and capacitively loaded traveling-wave electrode. (a) First high-performance thin-film lithium niobate electro-optic modulator using traveling-wave electrode^[15]; (b) thin-film lithium niobate electro-optic modulator using capacitively loaded traveling-wave electrode based on quartz substrate^[46]; (c) thin-film lithium niobate electro-optic modulator using capacitively loaded traveling-wave electrode based on silicon substrate^[47]

Fig. 7. Folded MZI-based modulators. (a) Folded thin-film lithium niobate electro-optic modulator using traveling-wave electrode based on poled MZI structure^[48]; (b) folded heterogenous thin-film lithium niobate electro-optic modulator using traveling-wave electrode and waveguide crossing structure^[49]; (c) folded thin-film lithium niobate electro-optic modulator using capacitively loaded traveling-wave electrode and waveguide crossing structure^[50]; (d) folded thin-film lithium niobate electro-optic modulator using capacitively loaded traveling-wave electrode and microelectrode crossing structure^[51]

Fig. 8. Single-polarization and dual-polarization IQ modulators. (a) Single-polarization thin-film lithium niobate IQ modulator using regular traveling-wave electrode^[52]; (b) dual-polarization thin-film lithium niobate IQ modulator using regular traveling-wave electrode^[53]; (c) dual-polarization thin-film lithium niobate IQ modulator using capacitively loaded traveling-wave electrode base on quartz substrate^[54]; (d) dual-polarization heterogenous thin-film lithium niobate IQ modulator using capacitively loaded traveling-wave electrode base on silicon substrate^[55]

Fig. 9. Resonator types of thin-film lithium niobate modulators. These types of modulators include PC modulator, ring modulator, 1×1 FP modulator, 2×2 FP modulator, SL modulator, and WBG modulator

Fig. 10. Photonic-crystal modulators and micro-ring modulators. (a) Photonic-crystal thin-film lithium niobate electro-optic modulator^[60]; (b) heterogenous micro-ring electro-optic modulator^[61]; homogeneous electro-optic modulator with (c) micro-ring resonator^[62] and (d) racetrack resonator^[63]

Fig. 11. Bragg grating resonator based modulators with different structures. (a) Single-mode 1×1 FP thin-film lithium niobate modulator^[64]; (b) multi-mode 2×2 FP thin-film lithium niobate modulator^[65]; (c) waveguide Bragg grating modulator^[66]; (d) slow-light thin-film lithium niobate modulator using coupled Bragg grating resonator^[67]

Fig. 12. Different types of modulators on thin-film lithium niobate. (a) Michelson interferometer modulator^[68]; (b) high-efficiency thin-film lithium niobate modulator using high-permittivity cladding^[71]; (c) Z-cut thin-film lithium niobate modulator^[72]; (d) ring-assisted Mach-Zehnder interferometer modulator on thin-film lithium niobate^[73]; (e) plasmonic lithium niobate Mach-Zehnder modulator^[74]; (f) thin-film lithium niobate modulator in topological interface states of a one-dimensional lattice^[75]

Fig. 13. Heterogeneous-bonded modulators on thin-film lithium niobate. (a) Hybrid silicon and lithium niobate modulator using BCB-bonded process^[76]; (b) hybrid silicon nitride and lithium niobate modulator using BCB-bonded process^[77]; (c) hybrid silicon and lithium niobate modulator using direct-bonded process^[78]; (d) hybrid silicon nitride and lithium niobate modulator using direct-bonded process^[79]

Fig. 14. Rib-loaded modulators and micro-transfer printing modulators on heterogeneous thin-film lithium niobate. (a) Heterogeneous silicon-on-lithium niobate modulator^[80]; (b) heterogeneous silicon nitride-on-lithium niobate modulator^[81]; (c) heterogeneous silicon nitride-on-lithium niobate modulator using micro-transfer printing^[90]

Fig. 15. Thin-film lithium niobate modulators under different working wavelengths. (a) Thin-film lithium niobate modulator at visible-light waveband^[92-94]; (b) thin-film lithium niobate modulator at near-infrared waveband^[95]; (c) thin-film lithium niobate modulator at 1064 nm waveband^[96]; (d) thin-film lithium niobate modulator at 2 μm waveband^[97]; (e) thin-film lithium niobate modulator at 3.39 μm waveband^[98]

Fig. 16. Multi-channel modulators on thin-film lithium niobate. (a) Four-channel transmitter using high-power distributed feedback laser on thin-film lithium niobate^[99]; (b) four-channel transmitter for QSFP-DD package on thin-film lithium niobate^[100]; (c) O-band CWDM transmitter on thin-film lithium niobate using titled multimode interferometer structure^[101]; (d) C-band CWDM transmitter on thin-film lithium niobate using multimode Bragg grating structure^[103]

Fig. 17. Applications of thin-film lithium niobate modulators. (a) Electro-optic frequency comb using non-resonator-based structure^[112] and resonator-based structure^[115]; (b) tunable^[117] and ultra-fast mode-locked^[119] laser on thin-film lithium niobate; (c) electro-optic isolator on thin-film lithium niobate^[120]; (d) microwave photoelectron processing engine on thin-film lithium niobate^[121]